About 4 years ago it was cold every day. The warmest day I hunted that year was 6, the coldest morning was -30 and breezy. I can honestly say I don't think I enjoyed hunting a single day of that season. Rather it was more of an obligation. That was a few years after I moved to MN and before I really understood what cold weather hunting was. I've since started trying to spend some money here and there on better cold weather stuff. This spring I finally broke down and bought an IWOM XT.
http://www.iwomouterwear.com/content/iwom-xt-1
Basically an improved version of the heater body suit, and they guarantee you won't get cold your first year or your money back. I'm pretty excited about it and hope it's as good as they say.

-20 last yr & killed a nice buck @4 10 in the afternoon on final day stayed warm in a built ground blind & a mister buddy heater seen alot of deer those 4 days & when you did see them they were all bunched up smallest herd i seen was 8 deer & largest herd i seen was aprox 20 & that when i got my buck was the last one coming thru

In November of 2013 the low on muzzleloader opener was -20 and the high was -15, it was one of those clear no wind days.

When I was young and full of pi$$ and vinegar I hunted when it was so cold that when I went to my stand in the dark my flashlight beam was frozen and I couldn't turn it off!
But seriously - probably about -10*F. Now if I get up and it's that cold, I just go back up stairs and nestle back in bed with my wife (or my lab) if my wife is in FL.

I don't allow my rotties in the bed. They BOTH snore louder than me!

Oh believe me - I wake either of them up a couple times a night to get them to stop snoring.

I hunt west central Wisconsin, One year it is 60% above than the next is - 20% I will go out to -5 but that is it The deer are smart enought to hunker down and don't move unless you kick them out.

??????? Deer don't move when it's super cold??????? Some of my best hunting was in really cold temps. Deer actually move MORE. Now if it's cold AND windy they will hunker down, usually in a pine thicket if they have one close, to stay out of that wind but usually they are up and feeding to stay warm. Also, if memory serves me correctly, deer need water the same as horses do when it's cold to help stay warm so you will see them a lot around water sources.